Four-Valve Boxer: BMW R100 Café Racer

Devil in the Details: 4V Krauser Airhead from Warkot Motorcycles… 

We see a lot of customized BMW air-cooled boxers (“Airheads”) coming out of workshops across the world, but this BMW R100 out of Poland’s Warkot Motorcycles immediately caught our attention. After all, those aren’t the standard BMW two-valve heads our eyes are accustomed to seeing.

“At first glance, you can see that this is not the usual BMW boxer due to the Krauser four-valve heads.” -Warkot Team

According to Motorcycle Classics, the Krauser four-valve head kit increased the 980cc R100’s power output from 70 to 82 hp. Compression was bumped from 8.2:1 to 10.2:1, and the kits came with Mahle pistons machined with pockets to match the valve layout.

Sold as race parts, the kits cost $1100 in 1983, and we’ve heard of period kits selling for more than double that. Real owners have described increases of 20 hp when matched with carbs and freer-flowing exhaust.

“I have taken it out on some isolated straight highways and there is no doubt the bike will pull clear through the redline in fifth gear if a braver owner was so inclined. I live well with the knowledge if not the actual experience. The legendary power of these heads would appear to be real.” -“R100 Super S”, BMW MOA Forums

But the four-valve heads aren’t the only element that sets this R100 apart. According to the “Vintage Brothers” who run Warkot Motorcycles — Maciej, Mateusz, and Michał — this is the first café racer build to come out of their workshop.

The trio thank their fathers for introducing them to motorcycles at an early age:

“We watched how they repaired them, and the sound of an engine coming to life and the smell of freshly poured petrol immediately made a smile on our faces.”

Today, they “rebuild and remake” motorcycles from the 1950s to 1980s out of their shop in Skierniewice, central Poland. For their first café racer build, they worked hard to get the lines right, rebuilding the subframe and adding a gas tank from the Slash 5 series.

The rear suspension was upgraded with YSS shock absorbers, and the Warkot crew upgraded the entire braking system.

“Front floating brake discs with a diameter of 320mm by Brembo were mounted on specially made adapters, while the calipers are six-piston Beringers, which are powered by the brake pump of the same company. The whole works on HEL steel braided wires. The rear brake, originally drum, has been changed to a disc from the R100RS.”

As for the engine, they did routine maintenance, replacing the clutch and various gaskets, and rebuilt the carbs and heads. The stainless exhaust is from Polish company PS Custom Bike, which emits a “beautiful bass sound,” though a set of screw-in DB killers keep the decibels down.

The headlight is an original unit, while the rear lights are minimalist 3-in-1 units, and the whole bike runs on a Motogadget system. Details such as front indicators, mirrors, and switches also come from Motogadget.

The Warkot crew put a major focus on high-quality materials, simplicity of construction, and “refinement of all details,” which is evident in this build.

The CNC upper yoke was designed to house the Motogadget Motoscope Pro tacho, and the headlight brackets, fender mounts, rear brake lever, and battery box were all handmade in the workshop.

The paint comes from the Porsche 356 color palette, and the saddle was trimmed with cognac leather.

While the build presented plenty of challenges, the “Vintage Brothers” are proud of the machine and the way it rides:

“The bike sounds great, accelerates great, and is very fun to ride. This build was a big challenge for us, but also gave a lot of joy and satisfaction.”

Follow the Builder

Web: warkotmotorcycles.pl
Instagram: @vintage.brothers
Photos: Rafał Gieleciński